Microbiological fouling has long been a problem with industrial aqueous systems. Aqueous industrial systems in which microbiological fouling can be a problem including industrial cooling water systems, in particular. However, microbiological fouling may be present and cause problems in any aqueous containing system.
Microbiological fouling may result from the presence of any of a variety of organisms in an aqueous system. Among the organisms that are important in microbiological fouling are a variety of algae, fungus, mold, and a variety of planktonic bacterial organisms. Such organisms may be introduced to aqueous industrial systems and intake water used in the system, from the environment, such as from the air, or from other process inleakage contamination sources, which could include, for example, an input of a component of foreign composition into the system or the introduction of contaminated equipment into a system.
Bacteria form a major component of microbiological fouling in aqueous systems. Bacteria can grow in almost any environment. Bacteria are loosely classified according to the environment in which they typically grow. Bacteria which are typically associated with microbiological fouling include anaerobic, facultative anaerobic, and aerobic bacteria. Anaerobic bacteria are particularly significant in microbiological fouling.
The production of a microbiological fouling film on a surface in a cooling system typically begins with the adsorption of an organic or inorganic layer on a surface of the aqueous system. Typically, the organic or inorganic layer has some nutritional value to microorganisms potentially forming a fouling film on the surface. The layer may nourish the microorganisms as well as facilitate their attachment.
Having a physical attachment point as well as a nutritional source provides a favorable location for the microorganisms to form a colony. Typically, less efficient motile organisms will drop out of the water first and attach themselves to the surfaces prepared with the nutritional layer. If the surface has been previously colonized and remnants of former colonies remain on the surface, the attachment of new organisms on a layer of a surface of a cooling system can be as rapid as fifteen minutes.
Upon attachment, the sessile microorganisms typically begin to produce polysaccharides or glycocalyx which help to secure their attachment to the nutritional layer and the surface upon which the layer is attached. Many microorganisms also produce glycocalyx which help to entrap nutrients and debris. Such glycocalyx can promote survival in low nutrient fluids and also enhance the microorganisms resistance to microbicide attack.
Often, an upper layer of aerobic bacteria is established on a surface, such as a metal surface of a heat exchanger, of the aqueous system first. The layer of aerobic bacteria typically provides a protective, nutrient, pH and temperature rich environment to allow lower oxygen tolerant bacteria to grow. Anaerobic bacteria may be transported into the aqueous system as spores and then encapsulated in a layer of the biofilm during the adhesion phase of the layer of aerobic bacteria.
A microbiological fouling layer may include a number of bacteria arranged in a colony having a complex structure. Such a colony may include a symbiotic matrix of acid producing, fermenting, iron oxidizing, and sulphate reducing bacteria. These bacteria may be protectively housed by a capping layer of glycocalyx/slime producing bacteria. The glycocalyx/slime layer may mediate the flow of organic, inorganic, and dissolved gas between the aqueous and gelatinous phases of the colony. However, the layer of bacteria may take any form and include any type of bacteria.
The deposition, attachment, and initial growth of a microorganism layer often is complete within about three to about five days. A mature colony of microorganisms, typically at least eight to ten cells thick, may be formed within about ten to about fourteen days. A microbial layer of this thickness may be sufficient to deplete the oxygen in the biofilm. Upon oxygen depletion, the conditions in the biofilm become favorable for the growth of anaerobic microorganisms, typically bacteria, to grow and thrive.
In typical industrial systems, a biofilm of microorganisms may continue to grow up to about 200 cells in thickness containing a maximum coverage to within about 3.5 to about 4 weeks. Such films may have a thickness of from about 300 up to about 500 microns. Even thicker films have also been reported. The thickness of the film may be increased, depending upon the amount of debris entrapped in the film and also the velocity of the water within the system. Such films typically include about 80%-98% water. A biofilm of just about 0.004 inch thick has been shown to have about one-quarter of the thermal conductivity of calcium carbonate scales of equivalent thickness. Accordingly, it can be seen how damaging a biofilm can be in relation to the functioning of an aqueous system.
Among the problems associated with microbiological fouling are a loss of efficiency, corrosion of parts of aqueous systems, an increase in friction in water flowing through the system and a resulting decrease in efficiency and increase in required energy to move water through the system, as well as an increase in slime and inorganic deposition. An increase in material in the aqueous system can also decrease the efficiency of the system and clog the system by taking up volume within the system.
Furthermore, even the first layer of bacteria or other microorganisms may influence the electrochemical kinetics of oxygen reduction. Whether in the initial stages of development or further developed, a microbiological fouling layer can cause a drop in the heat transfer coefficient or, in other words, a reduction in the efficiency of a heat exchanger in an aqueous cooling system. Microbiological films may also cause local increases in corrosivity that can result in premature failures of components of a cooling system or any other aqueous system.
Bacteria may also consume inorganic and/or organic nitrogen and phosphate base inhibitors. Additionally, the bacteria can affect corrosion by metabolizing inorganic or organic matter and producing corresponding acids through either respiration or fermentation, respectively. Formation of concentration cells from entrapment of corrosive ions and biofilm uptake by bacteria may also effect corrosion.
In addition to the above problems, bacteria can also effect the corrosion of surface in aqueous systems by anodic depolarization through noble metal deposition. Bacteria may also effect anodic depolarization through chelation uptake of the corroded metal by exopolymers in the biofilm. Furthermore, the bacteria may promote corrosive anaerobic bacterial growth through oxygen depletion. Acid producing bacteria may selectively dissolve zinc, magnesium, and calcium, resulting in cathodic corrosion control loss.
As stated above, other microorganism may be involved in microbiological fouling. For instance, cathodic depolarization may also take place due to daytime release of oxygen by algae. Also, fungi may dissolve protective coatings on surfaces in aqueous systems. Accordingly, in view of the above, it can be seen that bacteria are only one source of problems regarding microbiological fouling in aqueous systems and that other organisms can play a significant role in such fouling.
The detection and control of microorganisms has been well understood and practiced over the years. However, the fouling and corrosion associated with microbial growth continues to be an area of concern. According to normal industry practice, fouling due to microorganism growth is not detected until long after a reduction in heat transfer efficiency in cooling systems has occurred. However, by this time, a typical cooling system has been operating inefficiently for a long time and microbial growth is well advanced.
According to one method of detecting microbiological fouling, the measurement of a drop in dissolved oxygen on a probe membrane when a nutrient sugar is injected is indicative of microbiological fouling activity, provided further oxygen drop occurs upon subsequent nutrient injections. If no further oxygen occurs, then the fouling may be of the non-viable chemical type.
Another known method of detecting microbiological fouling is an electrochemical method that utilizes a probe that is cathodically polarized for a short period of time each day. However, with this method, measurements of generated current prior to polarization remain flat, without increases, if biofilms are not present. Further, according to this known method, measurements of the applied current prior to the end of the cathodic polarization will not increase with time if biofilms are not present. However, the presence of biofilms will be revealed by an increase both in applied and generated currents with respect to time from a base line value. Employing this method, detection times have varied from about 12 days on bench tests to about 14 to about 50 days in field tests, with a lag time of detection about 2.5 hours. This method does not provide information concerning loss of heat exchange performance.
Another electrochemical method merely employs the measurement of cathodic current and assumes the increase in cathodic current is due to microbial activity. Although at least some correlation between an increase in cathodic current and microbial activity exists, any degree of certainty that the change in current corresponds to biofouling requires that a peak current be reached and then a decline in current must occur.
These current changes have also been manually correlated to a biofilm build-up. The addition of a microbicide revealed a reduction in cathodic current that proved that the cathodic current was derived from microbiological activity. Unfortunately, no real time continuous fouling and/or tracking ability existed to allow for interpretation and/or differentiation of the film formed and whether it was actually microbiologically derived.
Once a fouling layer is detected and determined to be microbiological in nature, methods are used to control and eliminate the organisms responsible for the fouling. A variety of methods may be used to control microbial growth. Such methods may include changes to the conditions within the aqueous system and/or use of compounds having a microbicidal effect.
Changes to the environment within the aqueous system may include any change that creates an environment unfavorable for the existence of microorganisms. For example, it is well known that microbiological organisms in aqueous systems may be controlled by thermal shocks to the system.
Typically, the treatment used is tailored to the microbe(s) implicated in the fouling. The specific organisms involved in the microbiological fouling in any system may be cultured using typical culture methods. For instance, water may be sampled and cultures grown from the water. Surfaces within the system may be swabbed and the swabs cultured. Tests may be performed to determine the efficacy of various microbicides on the cultured organisms. A treatment plan may then be worked up using one or more microbicides.
According to one treatment protocol, the organisms causing microbiological fouling of aqueous systems are controlled using a variety of microbicides. For instance, microbicides such as chlorine or other known biocides may be injected into the system to control growth of microbiological organisms. The specific microbes requiring control affect the selection and method of application of the microbicides used.
A plurality of microbicides may be used simultaneously to obtain an additive or synergistic effect. Multiple microbicides may also be used to obtain more favorable results, particularly in using multiple microbicides having different methods of action on the microbes involved.
Although methods of detection of microbiological fouling are known, these methods suffer from shortcomings such as inefficiency, slowness, and uncertainty. Irreversible corrosion damage and operating problems may result from the delay in detecting microbiological fouling. Delay in detecting the problem may lead to, among other things, processing delay in the process being carried out by the aqueous system affected, inefficiency in the aqueous system, ineffectiveness of the system, and a loss of time, money, and energy.